yes, a missing texture will show that purple black blocked.
This is a light texture, it gives light so to speak.
And always funny he, how the npc's look at you in Hammer, i always feel myself watched by them, hahahaha.

Yeah. I have been looking through the source files of Hammer recently and have found that you can REPLACE... THE MISSING TEXTURE TEXTURE. Because they are a texture themselves, but still; the good ol' Source Engine, eh?